Flutter – How can I save the data of a document but changing one of the values

Using this I use the id of the document to identify the document that I want to modify but what if want to use a value inside of that document instead? For example:

dbUserDoc.collection('shoplist').get().then((querySnapshot) => {
            // ignore: avoid_function_literals_in_foreach_calls
            querySnapshot.docs.forEach((result) {
              final docPro = FirebaseFirestore.instance
                  .collection('products')
                  .doc(result.id); //here i use the document id but i want to use a value inside of that document
              print(result.id);
              docPro.get().then((DocumentSnapshot doc) {
                final data = doc.data() as Map<String, dynamic>;
                final int stockInt =
                    data['stock'] - int.parse(result['quantity']);
                final stock = <String, int>{"stock": stockInt};
                docPro.set(stock, SetOptions(merge: true));
              });
            })
          });